Symptoms
Tasks are not delegated to destination
Diagnoses
When a user sets their Out of the Office, the destination user does not receive any tasks upon their worklist. There are 8 pages of tasks as example.
If another user sets himself Out of the Office with the original user as a destination, the original user gets the tasks.
Resolution
Removing one of the old process instances from the worklist appears to solve the problem managed to delegate tasks to different users.